Why The WTC Towers Collapsed

Thick black smoke churns in poisonous clouds as the World Trade Center is swallowed by flames.

As CBS News Correspondent Bob Orr reports, the fire, an elaborate re-creation of the exact spot the north tower was hit, is at the heart of a two-year investigation into why the twin towers fell.

While the fire burns inside the lab, outside lie the skeletal remains of the fallen skyscrapers. Investigators, picked through tons of wreckage, matched scarred serial numbers and incredibly identified the exact columns struck by the planes.

"This particular piece was struck by the upper part of the fuselage," says Frank Gayle, a metallurgist at the National Institute of Standards and Technology. "We see how the bolts performed, everything along that line."

They've taken tiny samples of the steel and shot them through an air cannon to precisely calculate the damage done by the hijacked jets.

The tests strongly suggest the airplane strikes did not bring the towers down. And it wasn't the thousands of gallons of jet fuel the planes carried.

"The fuel itself got consumed in a matter of minutes," says Shyam Sunder, NIST's lead investigator in the WTC probe.

So the popular belief that all of this fuel made the building collapse inevitable, is not true, says Sunder.

To find out what why the buildings actually collapsed, scientists at the NIST re-created the 96th floor, complete with computers, carpets, furniture and paper. Then they set it on fire. Now, investigators are close to proving that it was this everyday office material burning in a fire fed by oxygen from continually breaking windows that ultimately caused the steel to give.

"Steel tends to soften or weaken and become more like Playdough when you heat it up," says Sunder.

Investigators will lay out their final report this fall. They'll issue sweeping recommendations aimed at making buildings better able to withstand catastrophic events and will call for improving evacuations with an eye toward surviving the unthinkable.